Among the feds’ arguments is one usually left unspoken: prohibition serves the interests of the pharmaceutical corporations. As expressed in the Solicitor General’s brief, “Excepting drug activity for personal use or free distribution from the sweep of the CSA would discourage the consumption of lawful controlled substances.” It would also undercut “the incentives for research and development into new legitimate drugs.” That’s as close as the government has come to acknowledging that wider cannabis use would jeopardize drug-company profits.

And that’s where the compelling government interest truly lies. (There’s no other logical reason to explain why the government has acted in such a bizarre manner regarding medical marijuana.)
Fred Gardner also explains the Rehnquist part of the equation (and he will not be at the oral arguments):

The absence of Chief Justice Rehnquist (undergoing treatments for cancer) works to Raich’s advantage. As a young lawyer in the Nixon White House, Rehnquist helped write the Controlled Substances Act. His questions during the OCBC [U.S. v. Oakland Cannabis Buyers Cooperative] oral argument were overtly hostile. And he’s considered results-oriented (fight the war on drugs) rather than principled (curtail the overarching commerce clause). Of course Rehnquist could still read the transcript and vote on the Raich case, even if he’s too sick to attend the oral argument. He could even write an opinion (or have his law clerks do so)… If there’s a 4-4 tie, the opinion of the 9th Circuit stands, but doesn’t become binding authority on the rest of the country.

Update: By the way, if anyone’s actually going to be attending the oral arguments on Monday morning, I’d love to hear from you.